Mix together the butter, sugar, egg, vanilla, and banana. Then add in the flour, baking soda, baking powder, and salt.
Put half of the mixture into a separate bowl and add the cocoa powder.
Grease the bottom of a 9 x 13 bar pan and pour the cocoa mixture in and spread thinly.
Spread the remainder of the batter on top of the cocoa mixture.
Sprinkle the top with chocolate chips. (You can use as little or as much as you'd like. I ended up doing less than the cup that the recipe calls for.)
Bake at 350F for about 30 minutes or until the top is browned and a toothpick comes out clean.
Once cooled, cut into bars or squares.
The mixture will be thick, just try and spread a thin layer across the bottom. It will rise up and make a beautiful thin bar. You can also use a smaller pan if you want a thicker bar, however, you will need to bake it longer to get the middle done.